You're correct about how it works.
I'd like to make such a change in the app, it's just ... complicated. The way IPS maps between tags and the content they apply to has no logical connection to that content type's search identifiers, which makes a generic solution all but impossible (as far as I'm aware). If we were only concerned about topics, I could say 'limit the tag search to the current forum' easily enough--but I don't know a way to do that and also have it work for content, and downloads, and every other tag-supporting app out there.